Recently, attention has been paid to a WiMAX (Worldwide Interoperability for Microwave Access) system as a mobile communication system. The WiMAX system covers a radius of about 50 Km with one wireless LAN access point and allows for a maximum communication speed of 70 Mbps.
FIG. 1 is a diagram showing the configuration of a WiMAX system. The WiMAX system comprises mobile stations 111, . . . , 1NQ, base stations 11, . . . , 1N, an ASN-GW (Access Service Network Gateway) 30, and an AAA (Authentication Authorization, Accounting) server 40.
The ASN-GW 30 manages and controls the hierarchically lower base stations 11, . . . , 1N and mobile stations 111, . . . , 1NQ that are connected to it.
The AAA server 40 performs authentication, authorization, and accounting of the mobile stations 111, . . . , 1NQ.
The mobile stations 111, . . . , 11p represent the mobile stations that are in the range of the base station 11, and mobile stations 1N1, . . . , 1NQ represent the mobile stations that are under entry to (i.e., in the range of) the base station 1N.
In the conventional WiMAX system, the ASN-GW 30 requests the AAA server 40 to authenticate a mobile station that attempts an entry to (i.e., to enter the range of) a base station. The ASN-GW 30 also sends the base station information, collected from the hierarchically lower base stations 11, . . . , 1N, to the mobile stations 111, . . . 1NQ as the neighboring base station information at a fixed time interval.
Patent Document 1 discloses a technology that allows a base station device to monitor the reception status of mobile stations, to detect a mobile station on which a call-loss or quality degradation occurs, and to cause the mobile station to handover to another base station device that has a carrier frequency different from that of the current base station device.    Patent Document 1: WO2004/112419